Field of the Invention
The invention lies in the electronics field. Specifically, the invention relates to a current-mode switching regulator for power supply, in particular for application in switched-mode power supplies.
Switched-mode power supplies are disclosed for example in Tietze and Schenk, "Electronic Circuits --Design and Application," Springer-Verlag 1991, p. 502. Switched-mode power supplies of this type usually comprise a rectifier, a power switch provided for the purpose of pulse width modulation, a filter, and also a regulator for controlling the power switch. An input voltage--for example a rectified power supply system voltage--is converted into a pulsed DC voltage with a variable duty ratio by the power switch acting as pulse width modulator. In this case, the pulse frequency may be set to be variable or fixed.
The task of the regulator is to keep the voltage at the output of the filter constant over a wide output current range. In other words, the regulator has to process the voltage at the output of the filter and the current at the output of the filter as input variables and form from them a control signal for the power switch. In this case, the timing ratio of the switch is influenced by the control signal.
Essentially three different control strategies exist for the regulation of such switching regulators: voltage-mode regulation, feedforward regulation and current-mode regulation. These control strategies are described for example in R. E. Tarter, Solid-State Power Conversion Handbook, Wiley Interscience, New York, 1993.
Current-mode regulation thereby constitutes a particularly elegant and effective type of regulation which, moreover, requires only comparatively small compensation capacitances. It is thus ideal for use in integrated circuits.
Current-mode switching regulators have two control loops. The inner control loop thereby serves to regulate the load current, while the outer control loop serves, together with the inner control loop, to regulate the output voltage. When the switching transistor is switched on, the voltage across a measuring resistor caused by the current rising in ramped fashion through the inductor coil of the switching regulator likewise has a ramped profile. When a desired voltage predetermined by the outer control loop is reached, the switching transistor switches off.
In current-mode switching regulators, it has been necessary heretofore, as described above, for the load circuit to have a measuring resistor in order to detect the current at the inductor. For the current detection, this measuring resistor is typically designed with a low resistance and, on account of its losses, is not suitable for use in integrated circuits. Such measuring resistors have to be connected externally to the integrated circuits in a cost-intensive manner, which requires considerable outlay in the course of fabrication.
However, the provision of a, typically, external measuring resistor turns out not only to be disadvantageous for cost reasons, but also reduces the efficiency of the switching regulator. Since the measuring resistor typically has to be connected externally to the integrated circuit, it is sometimes necessary, apart from a greater space requirement on the circuit board, to provide an additional output terminal on the integrated circuit.
Finally, the voltage drop measured across the measuring resistor has, for each switching period, an initial voltage spike produced by charge reversal of parasitic capacitances when the power switch is switched on. These undesirable voltage spikes have to be blanked out by additional measures--such as e.g. so-called "leading edge current blanking"--which are complicated in terms of circuitry, since the voltage spikes would otherwise impermissibly corrupt the actual measured value.